How to spread things out evenly
All the other answers are work-arounds when you want a single button solution like in Adobe Illustrator and others.

The solution you want is in the addon called Oscurart Tools. The link to the addon is here: http://wiki.blender.org/index.php/Extensions:2.6/Py/Scripts/3D_interaction/Oscurart_Tools

It comes with many features but the one you want is the second button from the bottom in the Object section.



Addon Download tip: The wiki page links to a list of separate file links that you have to download separately. Once you download them all, put them in a folder labelled "oscurart_tools" and then make a zip file from that. You can then import the addon from file this way for ease of use.